The binding of CO to a lacunar Fe(II) cyclidine complex is characterized by very negative values for the activation entropy and activation volume, whereas the reverse dissociation reaction is accompanied by small positive activation entropy and activation volume data. The constructed volume profile suggests that, in the transition state, CO disappears completely into the ligand pocket during partial Fe-Co bond formation, followed by a high-spin to low-spin transition on Fe(II) during which the metal center moves into the ligand plane and accounts for the subsequent volume decrease.